I linked Mail.app to my Gmail account and to my surprise it downloaded all of the 300000+ (!) messages that are on it (I looked in the Preferences for a way to limit the amount of messages downloaded but to no avail, if anybody has any directions on that please post them in a comment).

Since Mail.app refused to delete the messages in the mailbox I did not need, I wanted to check how much space on my disk all of those messages take.
I located the folder containing all my emails at /Users/gabriele/Library/Mail/V2/IMAP-*****/*****.mbox/*****.im.mbox which contains another folder with all of the messages I don't want but I'm forced to keep.

I showed the information for this folder but instead of the file size, it only said --:

At first I thought it might mean that it's still calculating so I let it sit, but it's been there for about 10 minutes now and it's still just two dashes.

1 Answer
1

The option is set individually per folder. If you want it applied by default, click the button at the bottom. Calculation folder sizes make take a long time though for big folders.

Alternatively you can open Terminal.app, type du -sm (followed by a Space), drag the folder you want to know to size of into the Terminal window and press Enter. Takes some time as well and gives you the size in Megabytes.